1. Field of the Invention
The present invention relates to a fuel cell formed by stacking a plurality of power generation cells in a stacking direction. The power generation cell includes a membrane electrode assembly, and a pair of metal separators sandwiching the membrane electrode assembly. The membrane electrode assembly includes a pair of electrodes and an electrolyte membrane interposed between the electrodes. A reactant gas passage and a coolant passage extend through the power generation cells in the stacking direction. Further, a coolant flow field for supplying a coolant is formed between metal separators of adjacent power generation cells.
2. Description of the Related Art
For example, a solid polymer fuel cell employs a membrane electrode assembly (MEA) which includes an anode and a cathode, and an electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly and separators sandwiching the membrane electrode assembly make up a unit of a power generation cell for generating electricity. In use, generally, a predetermined number of power generation cells are stacked together to form a fuel cell stack.
In the power generation cell, a fuel gas such as a gas chiefly containing hydrogen (hereinafter also referred to as the hydrogen-containing gas) is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating a DC electric current. A gas chiefly containing oxygen or air (hereinafter also referred to as the oxygen-containing gas) is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
In the power generation cell, a fuel gas flow field (reactant gas flow field) and an oxygen-containing gas flow field (reactant gas flow field) are formed in surfaces of the separators. The fuel gas flow field is formed on a surface of the separator facing the anode for supplying the fuel gas to the anode, and the oxygen-containing gas flow field is formed on a surface of the separator facing the cathode for supplying the oxygen-containing gas to the cathode. Further, a coolant flow field is formed between the power generation cells for allowing a coolant to flow along surfaces of the separators.
The separator may be made of a thin metal plate having corrugated surfaces fabricated by pressure forming for achieving reduction of the fabrication cost and the size. In this case, the reactant gas flow field and the coolant flow field are formed on both surfaces of the separator. The shape of the reactant gas flow field formed on one surface of the separator constrains the shape of the coolant flow field formed on the other surface of the separator. Therefore, it may not be possible to fabricate the desired shape of the coolant flow field.
In particular, in an internal manifold type fuel cell in which the reactant gas passages and the coolant passages extend through the outer region of the metal separators in the stacking direction, the coolant may not flow suitably between the coolant flow field and the coolant passages.
In an attempt to address the problem, as shown in FIG. 7, Japanese Laid-Open Patent Publication 6-218275 discloses a fuel cell block 1 formed by stacking membrane electrode assemblies 2 and separators 3 alternately. The membrane electrode assembly 2 includes a cathode 5a, an anode 5b, and an electrolyte membrane 4 interposed between the cathode 5a and the anode 5b. Each of the membrane electrode assemblies 2 is sandwiched between the separators 3, and contact plates 6 are provided between the membrane electrode assemblies 2 and the separators 3.
Each of the separators 3 includes plates 3a, 3b which are stacked together. Ridges of the plates 3a, 3b contact each other to form a cooling water chamber 7 between the plates 3a, 3b. 
A cooling water passage 8 extends through the fuel cell block 1 in the stacking direction indicated by an arrow X. The passage 8 is sealed by packings 9. The passage 8 is connected to the chamber 7 through an opening 7a in each of the separators 3. Therefore, in each of the separators 3, the cooling water supplied to the passage 8 flows from the opening 7a to the chamber 7. When the cooling water flows through the chamber 7, the cooling water cools the respective membrane electrode assemblies 2.
In Japanese Laid-Open Patent Publication No. 6-218275, the plates 3a, 3b of the separator 3 are made of thin plates. The chamber 7 connected to the opening 7a is formed between the plates 3a, 3b. Therefore, when a tightening load is applied to the fuel cell block 1 in the stacking direction, the plates 3a, 3b are deformed easily due to the opening 7a. Thus, it is difficult to suitably supply the cooling water to the chamber 7, and the sealing characteristics for preventing leakage of the reactant gases and the cooling water are lowered.